Waste water treatment systems used in the industry generally include, but are not limited to, the following treatment processes: grit removal, fine screening, flow equalization and primary clarification. The typical treatment processes are dependent on the velocity at which the waste water is moving through the system. Waste water, however, is not produced continually by humans, but instead is created in batch type processes, such as showering, flushing a toilet or operating a washing machine. Such water consumptive activities are generally repetitive resulting in daily, weekly, monthly and yearly diurnal flow patterns for a specific waste water treatment system. Accordingly, the volume of waste water produced, and the velocity of that waste water through the treatment system varies significantly throughout the day.
In the prior art, grit removal is generally performed in a grit chamber which is velocity sensitive. The most common methods to remove grit are by reducing the velocity of the influent flow so that the grit settles out, or utilizing a circular channel/tank. The circular channel/tank is a hydro-cyclone that causes the grit to settle in a sump, separating the organics from the grit so that they can move forward to the biological processes. The grit is then pumped out of the sump to a grit washer and then discharged to a dumpster for disposal at a landfill.
Fine screening is typically accomplished by placing a screen in an influent channel. The influent channel must have a minimum velocity of 1.25 feet per second to keep solids from settling out in the channel and a maximum velocity of 3.0 feet per second to keep solids from being forced through the screen. Such a flow is difficult to achieve due to the large variation in diurnal and pumped flow patterns.
Typical primary clarifiers are also velocity sensitive with the heavy solids going to the base of the clarifier where they are pumped to a digester, the floatable solids, grease and scum are trapped and skimmed off the surface and the neutral buoyant solids/clarified waste water exits the basin via an effluent weir. Primary clarifiers are typically large tanks designed for gravity settling and may include electrical drives, flights and chains, rack arms and paddles or suction tubes and sludge pumps.
Flow equalization typically occurs in a separate tank. The flow at the waste water plant is subject to travel times in the collection system, collection system design and pump station sizing. In general, larger collection systems use pump stations to lift the waste water to the treatment facility. The pumps are typically placed on variable-frequency drives in an attempt to provide a consistent uniform flow. The system of variable-frequency drives and pumps, however, fails in low and high flow conditions. The pumps must be designed for peak hourly flows and have minimal turn down capabilities.
Traditionally, waste water treatment plants have static bar racks or mechanically cleaned bar screens in channels at the entrance of the waste water into the treatment facility. These influent channels are typically constructed of concrete so as to last the life of the facility and are designed for specific waste water volumes, velocities (1 to 3 feet per second), and the insertion of specific screening and grit removal equipment.
The social behavior of flushing solids that should go to landfill such as baby wipes, diapers, swizzle sticks, condoms, tampon applicators, etc. creates issues for the operation of the waste water treatment facility. Many of these solids are neutrally buoyant or will float in the waste water. Elongated solids align with the flow and pass or are forced through the bar racks or mechanical screens because of the high flow. The flat sheet solids such as diapers and baby wipes cover the bar racks or screens causing the liquid level in the channel to rise and enter a bypass channel. These solids often end up creating issues in the treatment plant such as fouling pumps, valves, diffusers, and membranes ultimately ending up in the digester or sludge holding tank.
The increase in frequency and intensity of storm events producing exceptional precipitation combined with leaky sewage collection systems produces greater volumes of waste water delivered to the waste water treatment plant. Changes in societal behavior are not likely to occur. The cost to repair or replace the aged collection systems of developed nations is not fiscally achievable in the time frame needed. Therefore, the limited cross-sectional area of a channel requires an innovative approach to solve the above issues. The solution must be efficient in consideration of the goal to convert energy consumptive waste water treatment plants to sustainable resource recovery facilities where possible.
To accomplish the above, the influent channels must be replaced with tanks. Waste water design engineers and manufacturers of screening equipment recognize that high velocities and screening are in conflict. Yet the use of channels at the head of the waste water treatment process is still taught to engineering students today.